Splitting line layer at polygon's boundary of polygon layer using PostGIS

I have a line layer and a polygon layer in PostgreSQL 9.2, PostGIS 2.0. How can I split every line each time it crosses a polygon's boundary?

Answer

Alright, I can't test it on larger data set so it might not be fastest but should work.

If the polygons don't intersect (seems to be what you want) this will work. With ST_INTERSECTION(,) we can get lines inside of polygons (one line per row), with ST_Difference(,ST_UNION()) we can get a geometry colection that holds pieces of line that don't fit inside any of the polygons. So now we just have to put those two results in one format and UNION them:

    WITH
    lines_in_polygons AS(
      SELECT ST_INTERSECTION(line,polygon) as inter
      FROM
        line AS l,
        poly AS c
    ),

    diff AS (
      SELECT ST_Difference(l.line,ST_UNION(polygon)) geom FROM 
      line AS l
      JOIN poly AS p ON
      ST_INTERSECTS(l.line,p.polygon)
      GROUP BY l.line)

    SELECT ST_GeometryN(geom,n) 
    FROM diff AS d
    CROSS JOIN 

    (SELECT 
     generate_series(1,ST_NumGeometries(geom)) as n FROM diff) n
    UNION
    SELECT inter
    FROM lines_in_polygons;

If the polygons do intersect...I'll try tommorow.